Today, RF apparatuses used in mobile communications and broadcasting are rapidly becoming smaller and lighter. Coaxial resonators made of dielectric materials with high dielectric constant and low loss are extensively used as filters in RF apparatuses, which are required to be small and light. Such dielectric coaxial resonators are also made smaller by designing resonator shapes, for example, to change the characteristic impedance of the line stepwise, as well as using dielectric materials with large specific inductive capacity.
Next, a conventional dielectric filter is described. FIG. 7 is a cutaway sectional view of a conventional dielectric filter. As shown in FIG. 7, through holes 2A and 2B are created on a rectangular dielectric block 1, and the inside of the through holes 2A and 2B is metallized with inside conductors 4A and 4B. The periphery of the dielectric block 1 is metallized with an outside conductor 5. The inside conductors 4A and 4B are connected to the outside conductor 5 through one of openings in through holes 2A and 2B, respectively. An I/O electrode 7A is created by providing an isolated electrode on a part of the outside conductor 5. The I/O electrode 7A is electromagnetically coupled with the inside conductor 4A, and is connected to an external circuit. Another I/O electrode 7B (not shown in FIG. 7) is provided on a cut part, opposing the I/O electrode 7A. In the above configuration, a resonator is formed in the through holes 2A and 2B, and the dielectric filter shown in FIG. 7 operates as a two-step filter.
If the diameter of a through hole is stepped to configure a coaxial resonator with a larger hole diameter at the open-circuit end than that at the short-circuit end where the inside conductor and outside conductor are connected, capacitance for the outside conductor 5 is added to the line comprising the inside conductors 4A and 4B, enabling the shortening of the resonator length. In other words, the characteristic impedance of the resonance line formed by inside conductors 4A and 4B is stepped. By making the characteristic impedance at the open-circuit end lower than that at the short-circuit end, the resonator length can be made shorter than that of resonators with fixed characteristic impedance, thus allowing the overall size of the filter to be reduced.
However, in the conventional dielectric filter shown in FIG. 7, the resonator length can only be reduced to about half the size of a resonator with fixed characteristic impedance. Accordingly, no further reduction in size is feasible. At present, the conventional dielectric filter shown in FIG. 7 can be made several millimeters square for the 800 MHz band by using high dielectric material. This type of dielectric filter is often used in the RF section of mobile phones using this frequency band. For other RF apparatuses using lower frequency bands than 800 MHz, which require larger dielectric filters, helical filters are commonly employed instead of dielectric filter to reduce size. Since dielectric filters are inexpensive and easy to manufacture, and have several specific advantages such as low loss and high power resistance, a reduction in size would allow them to be employed in low-frequency band apparatuses.
The present invention aims to solve the problems described above and provide a small, light, and low-loss dielectric filter, compared to conventional ones, which are easily manufacturable and are particularly used at low frequency bands from VHF to UHF.